1. Field of the Invention
The present invention relates to an optical information processing apparatus with an optical head for irradiating an optical information recording medium with light, converting the light reflected by the optical information recording medium into a head signal, and outputting the head signal. The present invention relates also to a method of processing optical information.
2. Description of the Related Art
Optical discs called DVDs (Digital Versatile Disks) are commercially available as optical information recording media of high density and high capacity. Such optical discs have been used widely these days for recording media to record images, music, and computer data. Studies on optical discs for the next generation, i.e., optical discs having further improved recording density, have proceeded in many facilities. Such next-generation optical discs are expected as recording media for replacing videotapes used for the currently-dominating VTRs (Video Tape Recorders), and the development is pursued at a feverish pitch.
An available technique for improving the recording density of an optical disc is to reduce the spot formed on a recording surface of an optical disc. Such a spot can be reduced by increasing the numerical aperture of light radiated from the optical head and decreasing the wavelength of the light.
However, a spherical aberration caused by an error in thickness of a protective layer formed on the optical disc will be increased rapidly when the numerical aperture of light radiated from the optical head is increased and the wavelength of the light is decreased. Therefore, a means for compensating the spherical aberration is required. The following description is about a conventional optical information processing apparatus having a means for compensating the spherical aberration.
FIG. 15 is a block diagram showing a configuration of a conventional optical information processing apparatus 90, and FIG. 16 is a block diagram for explaining a configuration of an optical head 5 provided in the conventional optical information processing apparatus 90. The optical head 5 in the optical information processing apparatus 90 has a semiconductor laser 123. A light beam 122 emitted from the semiconductor laser 123 passes through a prism 124, and it is collimated by a focusing lens 13 so as to be a substantially parallel light beam.
The light beam collimated by the focusing lens 13 passes through a concave lens and a convex lens provided in a spherical aberration compensator 7 and the light beam is reflected by a mirror 14. The light beam reflected by the mirror 14 is converged by an object lens 9 so as to form a spot on a recording surface formed on an optical disc 6, and reflected by the recording surface so as to form reflected light 33. The reflected light 33 passes again through the objective lens 9, and it is reflected by the mirror 14. Then, the light passes through the spherical aberration compensator 7, and it is focused by the focusing lens 13. After being focused by the focusing lens 13, the light 33 is reflected by a prism 124, and it passes through a hologram 115 provided for detecting a spherical aberration and also a cylindrical lens 116 provided for detecting a focal position so as to enter a photodetector 117.
The photodetector 117 generates a head signal on the basis of the reflected light 33 as incident light, and outputs the head signal into a preamp 18. The preamp 18 generates and outputs a focusing error signal FE according to astigmatism on the basis of the head signal outputted from the photodetector 117 provided in the optical head 5. Moreover, as disclosed in Tokuhyo-2001-507463 (published Japanese translation of PCT international publication for patent application), the preamp 18 detects separately a focusing error signal at the inner radius of the reflected light 33 and that of the rim of the reflected light 33, and generates a spherical aberration error signal SAE on the basis of the difference between the focusing error signals and outputs spherical aberration error signal SAE.
The focusing error signal FE outputted from the preamp 18 is inputted into a signal-amplitude instrument 20 via a switch 28. The signal-amplitude instrument 20 measures an amplitude of the focusing error signal FE and outputs the measurement result as a detection signal FEpp into an amplitude-maximum probe 21. The amplitude-maximum probe 21 outputs a spherical aberration compensating signal ΔSAE into an adder 26 so that the detection signal FEpp has a maximum amplitude.
The amplitude-maximum probe 21 searches for the spherical aberration, using the detection signal FEpp as the evaluation value so as to obtain a maximum detection signal FEpp. An example of the methods for searching for an optimum spherical aberration as described above includes varying the spherical aberration compensating signal ΔSAE slightly in order to slightly fluctuate the spherical aberration, checking a fluctuation of the amplitude of the detection signal FEpp at that time, and varying the spherical aberration compensating signal ΔSAE for increasing the detection signal FEpp.
Since a switch 27 is in an OFF-state, the adder 26 outputs the spherical aberration compensating signal ΔSAE from the amplitude-maximum probe 21 into a spherical aberration controller 12. The spherical aberration controller 12 outputs a control signal into a spherical aberration compensating actuator 8 provided in the spherical aberration compensator 7 of the optical head 5, on the basis of the spherical aberration compensating signal ΔSAE outputted from the adder 26, in order to vary a divergence of the light beam by varying spacing between two lenses provided in the spherical aberration compensator 7 and compensate the spherical aberration caused by an error in thickness of a protective layer formed on the optical disc 6.
The preamp 18 generates a reproduction signal RF by amplifying the head signal outputted from the optical head 5, and outputs the reproduction signal RF into a jitter detector 4. The jitter detector 4 measures jitter of the reproduction signal RF outputted from the preamp 18, and outputs the measurement result as a jitter detection signal JT into a minimum-jitter probe 91.
Here, the term ‘jitter’ denotes a physical quantity representing a time delay of an information transition for a reproduction signal. The jitter has a close relationship with an error rate representing the probability of error occurrence at the time of reading information from the optical disc. Therefore, the jitter is used as an evaluation value for controlling in the optical information processing apparatus.
The minimum-jitter probe 91 searches for a focal position having a minimum jitter value by using a technique similar to the above-described case where the amplitude-maximum probe 21 is used, and outputs a focal position compensating signal ΔFE into the adder 25. The switch 28 is turned to the adder 25, and the focusing error signal FE from the preamp 18 is outputted into the adder 25. The adder 25 performs addition of the focusing error signal FE outputted from the preamp 18 and the focal position compensating signal ΔFE outputted from the minimum-jitter probe 91, and outputs the result into the focusing controller 11. On the basis of the result of addition outputted from the adder 25, the focusing controller 11 outputs a control signal into a focusing actuator 10 provided in the optical head 5. On the basis of the control signal outputted from the focusing controller 11, the focusing actuator 10 drives the objective lens 9 along with a direction perpendicular to the optical disc 6 in order to control the focal position of the light beam converged on the optical disc 6. Accordingly, a focus control is performed.
Then, the switch 27 is turned from an OFF-state to an ON-state. Into the adder 26, the amplitude-maximum probe 21 outputs the spherical aberration compensating signal ΔSAE that maximizes the amplitude of the focusing error signal FE stored in advance of the performance of the focus control. The adder 26 performs addition of the spherical aberration SAE outputted from the preamp 18 and the spherical aberration compensating signal ΔSAE outputted from the amplitude-maximum probe 21 and outputs the result into the spherical aberration controller 12. On the basis of the addition result outputted from the adder 26, the spherical aberration controller 12 outputs the control signal into the spherical aberration compensating actuator 8 provided in the spherical aberration compensator 7 of the optical head 5. The spherical aberration compensating actuator 8, on the basis of the control signal outputted from the spherical aberration controller 12, varies spacing between two lenses provided in the spherical aberration compensator 7 and varies the divergence of the light beam in order to compensate the spherical aberration caused by an error in thickness of the protective layer formed on the optical disc 6.
In this manner, an optical disc apparatus according to the conventional technique compensates the spherical aberration first, and then searches for a focal position that minimizes the jitter value.
However, a recent study by the inventors clarified that the jitter may not be converged to its minimum value in the thus configured optical information processing apparatus.
FIGS. 17A–17C are graphs showing the relationship between a wave front aberration and a distance from a center of a light beam. The x-axis in each graph indicates a distance from a center of a light beam radiated from the optical head 5 onto the optical disc 6, and the y-axis indicates a wave front aberration. The wave front aberration is used for evaluating optical characteristics of the optical head since it has a close relationship with jitter.
FIG. 17A shows a relationship between a wave front aberration and a distance from a center of a light beam, where the light beam has a focal position at a location displaced by some degree from the recording surface formed on the optical disc along with a direction perpendicular to the surface of the optical disc. As shown in FIG. 17A, a curve indicating a relationship between the wave front aberration and a distance from a center of a light beam makes a quadratic curve in the case that the focal position of the light beam is displaced from the recording surface.
FIG. 17B shows a relationship between a wave front aberration and a distance from a center of a light beam when a spherical aberration is provided by 20 mλ using the spherical aberration compensator 7 for a case that the focal position is displaced as shown in FIG. 17A. As clearly indicated by the curve in FIG. 17B, the total wave front aberration is increased in a comparison with the total wave front aberration shown in FIG. 17A.
FIG. 17C shows a relationship between a wave front aberration and a distance from a center of a light beam when a spherical aberration is provided by −20 mλ using the spherical aberration compensator 7 in a case that the focal position is displaced as shown in FIG. 17A. As clearly indicated by the curve in FIG. 17C, the total wave front aberration is decreased in comparison with the total wave front aberration shown in FIG. 17A.
As described above, a total wave front aberration is increased for the case of FIG. 17B while it is decreased for the case of FIG. 17C even when providing spherical aberrations that are identical in the absolute value. This indicates that the focal position and the spherical aberration are influenced by each other, and that the focal position and the spherical aberration are under an influence of jitter.
In the above-described conventional optical information processing apparatus, the spherical aberration and the focal position are searched separately, for example, by searching for a spherical aberration that maximizes an amplitude of a focusing error signal and then searching for a focal position that minimizes a jitter value.
However, as described above, both the focal position and the spherical aberration influence jitter. Therefore, when the focal position and the spherical aberration are searched separately, a convergence result in the searches may vary depending on the initial focal position and the initial spherical aberration. This may result in failures in obtaining a result in a search to find a true minimum value of the jitter. When a searched jitter value is shifted from the true minimum value, reproduction signals will deteriorate. Moreover, either record information or address information recorded on the optical disc may not be read normally. Furthermore, information may not be recorded accurately since recording on the optical disc is carried out in a state that the spot of the light beam is spread.
The present invention aims to solve the above-described problems, and the object is to provide an optical information processing apparatus for obtaining a high quality signal reproduced from an optical disc, and a method of processing optical information.